The present invention relates to circuits for interconnecting computer networks.
Computer networks consist of a number of computer systems coupled together with a bus, rings or other medium, so that they can communicate with each other. Examples of such a system are the Xerox ETHERNET System and the IBM Token Ring Systems. Oftentimes, it is desirable to connect two such networks together. FIG. 1A shows such a configuration in which a number of hosts 12 are connected together with a bus 14 in a network A and a number of hosts 16 are connected together with a bus 18 in a network B. Buses 14 and 18 are coupled together to interconnect the networks with a repeater 20.
When more than two networks are coupled together, it is often desirable to replace the repeater with a circuit which can route a communication packet from a host computer in one network to a computer in another network. These circuits are often called bridge circuits. A system interconnected using bridge circuits is shown in FIG. 1B. A number of bridge circuits 22-30 interconnect a number of networks 32-42. Networks 32-42 each consist of a bus or other medium, and several attached hosts, similar to networks A and B, in FIG. 1A. Links 53 and 55 are shown as simple links. However, they could be full networks, with hosts connected to them.
These bridge circuits receive a data packet from one network and send it out an appropriate port of the bridge circuit so that it will be directed to the appropriate destination network. For instance, a data packet from a computer in network 36 could be transmitted to bridge 24. Bridge 24 would then determine that network 38 is the destination. Accordingly, the packet would be sent out a port 44 of bridge 24 and not a port 48. Bridge 22 would receive the packet and ignore it while bridge 26 would receive the packet and direct it out its port 50. A problem arises if there were to be a connection such as that shown in phantom as line 52, which would cause a loop. In this instance, bridge 24 could send the data packet out port 48 and route it through network 42, bridge 30 and bridge 28 to bridge 26 in the example just given. To avoid such loops, data path 52 will be ignored by bridges 28 and 30 if it is so connected. An example of such a bridge circuit is described in U.S. Pat. No. 4,597,078 to Kempf.
If there are multiple paths to a destination network, the prior art required bridge circuits to disable links until no loops remained in the system.